1. Field of the Invention
This invention relates to apparatus and method for changing the temperature of elongated moving webs, and in particular to an apparatus and method for uniformly heating or cooling a coated web.
2. Description of the Prior Art
U.S. Pat. No. 3,065,098 teaches cooling a moving web by conveying it on a supporting cushion of air flowing from perforations in a plurality of hollow stationary cylindrical tubes. Air is drawn downwardly into the space between adjacent tubes by a suction device which establishes a suction on the web, causing it to move in an undulated path. The cooling effect is produced solely by transfer of heat from the web to the air cushion, and thus is somewhat limited in effectiveness.
U.S. Pat. No. 3,738,018 describes guiding a moving web through a dryer by spaced pairs of guide rollers in engagement with one surface of the web. A suction device between each pair of rollers maintains a predetermined angle of contact of the web with the rollers. The drying effect results from contact of the web with the heated rollers and hot air in the dryer.
It is known in the cooling of webs coated with photographic emulsions to employ a suction device in cooperation with closely-spaced chill rollers. The suction device produces a downward air flow to assist in producing a predetermined angle of contact of the web with the chill rollers. The rollers may be idler rollers turned by the movement of the web thereon. A disadvantage of this known type of chilling device is that the web does not change temperature uniformly across its width resulting in a higher temperature at its width resulting in a higher temperature at its center portion and a lower temperature at its edge portions. This non-uniformity is believed to result from air flowing under the web from edge to center which has a greater cooling effect on the temperature of the edge portion of the web than on the temperature of the center portion. This non-uniform temperature gradient across the web results in variations in web or web coating properties, more particularly photographic coating properties, widthwise across the web.
In addition, where contact with the rollers is important for heat transfer, the length of web "chilled" during the period of time that the rollers achieve an equilibrium temperature following machine startup must be discarded due to non-uniform heat transfer. This constitutes extra cost to the manufacturer. This invention reduces the time required to reach temperature equilibrium and the footage of wasted web.